Molecular Fluorescence and Phosphorescence (5) Flashcards
What is photoluminescence?
process where molecules are elevated to excited state; excess energy emitted as radiation of wavelength different than wavelength of absorption; excitation/de-excitation process involving photons
de-excitation process that occurs after excitation by photons
photoluminescence
de-excitation process that occurs after excitation by high energy particles
radioluminescence
de-excitation process that occurs after excitation by chemical process
chemiluminescence
what is the total spin quantum number S?
sum of net spin
What is S for uncharged molecules?
0
How is multiplicity calculated?
2S + 1 = x where x is like singlet, triplet, etc
What is a singlet state?
multiplicity of one
What is S0?
ground
What is S1?
excited state 1
What is S2?
excited state 2
What is a triples tate?
formed when an excited electron reverses its spin
what is the quantum mechanical selection rule for single and triplet states?
ΔS=0
What is the time scale for electronic absorption?
10^-15 seconds
What is a Jablonski diagram?
qualitative depiction of absorption and emission processes
What is vibrational relaxation?
molecule rapidly and nonradiactively dissipates excess vibrational energy as heat by collision with solvent molecules
What is the time scale for vibrational relaxation?
10^-12 seconds
What is internal conversion?
molecule intermolecularly passes from original excited state to a lower one of the same multiplicity
What is the time scale for internal conversion?
10^-12 seconds
What is fluorescence
process of photon emission for de-excitation S1 to S0
How does the wavelength of fluorescence emission compare with wavelength of initial absorption and why?
at lower energy and longer wavelength because of vibrational relaxation
What is the 0-0 transition?
fluorescence and absorption between two electron states that have at least one transition of the same energies; between v” = 0 S0 and v’ = 0 S1
What is the time scale for fluorescence?
10^-9 seconds
What is external conversion?
energy transfer between excited molecules and solvents
How does external conversion differ from internal conversion?
external is de-excitation between excited molecule and other solvents, internal is lowering from original excited to a lower excited state of the same multiplicity (S2 to S1)
What is intersystem crossing?
molecule in excited state may change spin multiplicity from single to triplet
What is the time scale for intersystem crossing?
varies but can be 10^-10 seconds with heavy or paramagnetic molecules
What is phosphorescence?
From triplet, molecule rapidly relaxes to lowest vibrational level
From which spin state does phosphorescence occur?
T1 to S0
What is the time scale of phosphorescence?
10^-3 to 10 seconds
Why is phosphorescence frequently carried out at liquid nitrogen temperatures?
cannot be seen in fluid solutions at room temperature because there are many deactivation processes that have faster rate constants
What is quenching?
non radiative process that depopulates S1 or T1 levels and competes with fluorescence and phosphorescence
What governs the observed intensities in electronic processes?
overlap of vibrational wave functions in upper and lower states according to frank-condos principle (about geometry changes)
What is the stokes loss/shift?
difference in energy between position of maximum emission and 0-0 transition
what does stokes shift measure?
change in geometry of equilibrium configurations of ground and excited states
What kinetic rate law describes fluorescence?
pseudo first order kinetics
What is the fluorescent quantum yield?
fraction of total number of photons absorbed that result in fluorescent emission
How is fluorescent quantum yield defined?
total number of fluorescent photons emitted / total number photons absorbed
How is the steady state fluorescent lifetime related to the kinetic rate constants of the de-excitation process?
inverse
How is the fluorescent quantum yield related to the steady state lifetime?
quantum yield = steady state lifetime divided by radiative lifetime
